This invention relates to curable elastomer-modified epoxy resin-curative compositions and to coatings made from such compositions. In a specific aspect, the invention relates to chemically-modified phenolic curing agents employed to enhance the properties of epoxy resin powder coating formulations.
Rubber-modified epoxy resins are well known. Rubber modification, by blending or co-reacting elastomeric compounds, is employed to improve such physicial properties of epoxy resins as toughness, low profile characteristics, flexibility, and adhesion, producing resins for improved corrosion and chip resistant coatings.
Powder coatings having corrosion and chip resistance are used in the automotive industry as primer coatings. Automobile and truck bodies, particularly wheel wells and lower portions of the bodies such as rocker panels, are susceptible to chipping by sand and gravel. Some of the chipping may eventually result in rusting. Since the chip-resistant primer coating is often placed between an electrodeposited primer layer directly on the metal and an outer topcoat layer, a chip-resistant primer also requires good intercoat adhesion. In some areas, such as wheel wells and underbodies, the chip resistant primer may not be topcoated, and thus must demonstrate good resistance to the effects of exposure.
Numerous organic solvent-based coating compositions useful as chip resistant primers have been proposed, as for example, in U.S. Pat. Nos. 4,581,424, 4,602,053, 4,608,313, and 4,614,683. However, these coating compositions have a high volatile organic compound (VOC) content, which poses a problem for industries attempting to meet governmental guidelines on VOCs.
Powder coatings, which have the advantage of very low VOC, have been suggested, as in U.S. Pat. No. 4,804,581, which discloses a coating composition comprising an elastomer-modified epoxy derived from a polyepoxide and from about 5 to about 35 weight percent of a functionally-terminated diene-containing polymer and a carboxy-functional polyester. Although the '581 disclosure teaches that epoxy resins used as powder coatings may be modified with up to about 35 weight percent functionally-terminated diene-containing polymer, it has been found that high levels of rubber modifier in the epoxy resin contributes to stability and viscosity problems in the composition.
It is therefore an object of the invention to provide an epoxy resin composition useful in coating applications. It is another object of the invention to provide curing agents for epoxy resin-based powder coating compositions. It is yet another object of the invention to provide low-viscosity rubber-modified powder coating formulations.